This application claims the benefit of Japanese Patent Application No. JP 2001-339174 filed on Nov. 5, 2001, and Japanese Patent Application No. JP 2002-082347 filed on Mar. 25, 2002, both in the Japanese Patent Office, and the disclosures of the above applications and Japanese Patent Application No. 2002-233114, filed on Aug. 9, 2002, in the Japanese Patent Office, are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a cryocooler for forming a cryogenic temperature state. More particularly, the present invention relates to a pulse tube cryocooler using a Stirling cycle and including a pulse tube and a regenerator.
2. Description of the Related Art
Since a cryocooler using a Stirling cycle can obtain cryogenic temperatures by repeatedly compressing and expanding a working gas, it has become widely used in cooling operations, such as for cooling of superconducting elements, refining and separation of gases, infrared ray sensors, or the like.
The operation principle of a Stirling cryocooler, using this Stirling cycle, can be more fully explained using FIGS. 2 and 3. FIG. 2 is an explanatory view showing the outline of a refrigeration cycle, and FIG. 3 is a diagram showing a cycle of rising and falling of a compression piston and a displacer in accordance with a refrigeration cycle.
As illustrated in FIG. 2, a Stirling cryocooler 20 can include a compressor 21 having a compression piston 22, a regenerator 23 having a regenerating agent, a displacer 24 forming an expansion chamber 25 and a compression chamber 28, a cooling part 26 formed between the expansion chamber 25 and the regenerator 23, and a heat radiation part 27 formed around the compression chamber 28. A working gas is sealed under high pressure in a hermetically sealed flow passage constituted by these members, and the compression piston 22, of the compressor 21, and the displacer 24 are reciprocated with a phase difference therebetween.
In FIG. 3, a solid line 22a represents a rising and falling of the compression piston 22, and a solid line 24a indicates rising and falling of the displacer 24. Solid line 29 represents the total volume change in the cryocooler by the rising and falling of the compression piston 22.
As is seen in the volume (P)xe2x80x94pressure (V) diagram illustrated in FIG. 2, the Stirling cycle encompasses a process having two isothermal changes and two constant-volume changes.
A process from xe2x80x9caxe2x80x9d to xe2x80x9cb,xe2x80x9d illustrated in portion (A) of FIG. 2, is an isothermal expansion process, where the compression piston 22 goes down from a top dead point to a bottom dead point so that the working gas in the expansion chamber 25 is expanded, heat Qc is absorbed from the cooling part 26, and cooling is performed.
A process from xe2x80x9cbxe2x80x9d to xe2x80x9cc,xe2x80x9d illustrated in portion (B) of FIG. 2, is a constant-volume heating process, where the displacer 24 goes up from the bottom dead point to the top dead point, so that the fluid in the expansion chamber 25 is pushed out and into a space at the side of the compression chamber 28 through the regenerator 23 and that pressure is raised.
A process from xe2x80x9ccxe2x80x9d to xe2x80x9cd,xe2x80x9d illustrated in portion (C) of FIG. 2, is an isothermal compression process, where the compression piston 22 goes up from the bottom dead point to the top dead point, so that the working gas is fed into the compression chamber 28, and is isothermally compressed by radiating heat Qh at the heat radiation part 27.
Finally, a process from xe2x80x9cdxe2x80x9d to xe2x80x9ca,xe2x80x9d illustrated in portion (D) of FIG. 2, is a constant-volume cooling process, where the displacer 24 goes down from the top dead point to the bottom dead point, such that the fluid in the compression chamber 28 is pushed out to the side of the expansion chamber 25 through the regenerator 23, the pressure falls, and the cycle is ended.
In this cycle, as shown by the solid lines 22a and 24a of FIG. 3, the phase difference between the compression piston 22 and the displacer 24 is set to approximately 90 degrees.
As stated above, in the Stirling cryocooler, the compression piston is displaced by mechanical power, so that the pressure of the working gas in the sealed space is changed. The working gas in the expansion chamber is expanded, to cool, using the displacer moving in synchronization with the periodic change of this pressure. Therefore, a high heat efficiency can usually be achieved.
On the other hand, as a cryocooler using this Stirling cycle, a pulse tube cryocooler shown in FIG. 4 is also known.
Pulse tube cryocooler 10 is provided with a compressor 11 to repetitively feed and suction a working gas, a regenerator 13, coupled to the compressor 11 through a heat radiation part 12 and having a regenerating agent, a pulse tube 15, coupled to the regenerator 13 through a cooling part 14, and a buffer tank 18 coupled to this pulse tube 15 through a heat radiation part 16 and an inertance tube 17.
A working gas such as helium, nitrogen or hydrogen can be sealed under high pressure in a hermetically sealed space of this pulse tube cryocooler 10. Then, similarly to the foregoing Stirling cryocooler, expansion and compression of the working gas is repeated by the compressor 11 to form a pressure amplitude.
Here, in the pulse tube cryocooler 10, the working gas 30 in the pulse tube 15 oscillates minutely in the flow passage, such that it functions as the displacer in the foregoing Stirling cryocooler example. Accordingly, the working gas 30 can be made to work by controlling the phase of the displacement of the oscillating working gas 30 and the pressure displacement. Heat Q1 and Q3 are radiated from the heat radiation parts 12 and 16, heat Q2 is absorbed in the cooling part 14 which becomes a cold head of the cryocooler, such that a cryogenic temperature state is formed. The inertance tube 17 and the buffer 18 serve to control the phases of the displacement of the oscillating working gas 30 and the displacement of the compression piston.
In this pulse tube cryocooler, the displacer installed in the Stirling cryocooler is not necessary, and instead of the displacer, the high pressure gas is minutely oscillated so that the working gas can be compressed and expanded. Therefore, there are no movable parts in the low temperature portion. Thus, since mechanical oscillation does not exist at a cooling head, an equipment structure becomes simple, resulting in high efficiency and reliability.
The output (cryocooler output) in the above pulse tube cryocooler is determined by a difference between an output (hereinafter referred to as an indicated cryocooler output) in proportion to the product of a pressure amplitude and a flow amplitude in the inner area of the pulse tube, and various heat losses generated inside the cryocooler. This is represented by the following relation.
(refrigeration output)=(indicated refrigeration output)xe2x88x92(heat loss)
In order to improve the cooling efficiency of the pulse tube cryocooler, an understanding of the following two desired results become important: (1) to increase the indicated refrigeration output by efficiently transmitting the pressure amplitude given by the compression piston of the compressor into the pulse tube, and (2) to reduce the heat loss due to heat conduction in respective structural units, especially in the regenerator.
First, with respect to the regenerator, in order to reduce the above heat loss, it becomes necessary to reduce heat conduction through the structure of the regenerator, due to the temperature difference between the heat radiation part 12 and the cooling part 14, illustrated in FIG. 4. That is, potential heat of the working gas supplied and exhausted from the compressor 11 is temporarily stored, and the inflow of heat from the heat radiation part 12 of the high temperature side to the cooling part 14 of the low temperature side through the working gas is reduced.
For this purpose, it is conceivable that the heat capacity can be increased by increasing the inner volume of the regenerator 13, or heat resistance can be increased by elongating the regenerator 13 in an axial direction.
However, on the other hand, from the viewpoint of the indicated refrigeration output, in order to efficiently transmit the pressure amplitude generated in the compressor 11 to the pulse tube 15, it is necessary that the pressure loss of the regenerator 13 be small. Accordingly, from this viewpoint, it is also preferable that the length of the regenerator 13 be short.
Accordingly, it is conceivable that with respect to the regenerator 13, the inner volume, length, etc., of the regenerator be optimized so as to satisfy the above contradictory requests.
On the other hand, also with respect to the pulse tube 15, in order to reduce the above heat loss, the heat conduction through the structural member of the pulse tube is lowered, caused by the temperature difference between the heat radiation part 16 and the cooling part 14. The heat resistance in the axial direction in the pulse tube 15 should be large, and therefore, the pulse tube 15 can be made long in the axial direction to increase heat resistance.
However, similarly to the regenerator 13, in view of securement of the pressure amplitude to increase the indicated refrigeration output, with respect to the pressure amplitude from the compressor 11, it is necessary to keep the pressure amplitude in the pulse tube 15 at a large value. Thus, from the viewpoint of the pressure loss, the length of the pulse tube 15 should be short. Accordingly, also with respect to the pulse tube 15, the inner volume, length, etc. of the pulse tube should be optimized so as to satisfy the contradictory requests at the same time.
Since the above regenerator 13 and the pulse tube 15 are united to make up the cryocooler, it is conceivable that there is an optimum range also with respect to the relation of volume, length, etc., of the regenerator 13 to those of the pulse tube 15, and it is conceivable that the efficiency of the cryocooler is greatly influenced by this optimum range.
Accordingly, one problem of conventional systems is their lack to provide a pulse tube cryocooler having high refrigeration efficiency by optimizing the above respective contradictory conditions.
A pulse tube cryocooler is superior in that there is no mechanical oscillation, allowing for a simplification of equipment structure and improvement of reliability. However, as noted above, the conventional pulse tube cryocooler is flawed since the refrigeration output is apt to be changed by an installation posture, that is, a relative positional relation between the regenerator and the pulse tube at the time of installation. It is therefore necessary to form a structure which is not affected much by this installation posture.
As set forth above, a refrigeration output of the pulse tube cryocooler can be represented by the following expression.
(refrigeration output)=(indicated refrigeration output)xe2x88x92(heat loss)
In the xe2x80x9cheat lossxe2x80x9d of this expression, noting that heat loss is affected by the installation posture of the cryocooler, there is such a heat loss that a sealed working gas generates convection in the pulse tube inner area and the regenerator inner area, with heat entering into the cold head from the high temperature end.
Since the cold head is at a cryogenic temperature of, for example, approximately 70 K, and the high temperature end is normally at a normal temperature (approximately 300 K), the density of the working gas varies greatly between the cold head and the high temperature end. Therefore, convection by gravity occurs, and the degree of this convection can be affected by the installation posture. Thus, the heat loss by this convection is also affected by the installation posture.
Hereinafter, the influence of the installation posture will be described with a pulse tube, as an example.
In an installed state, the cold head of the pulse tube is positioned to be higher than the high temperature end, whereby the temperature of the working gas in the inner space of the pulse tube has such a state that the temperature of the lower part in contact with the high temperature end is high as compared with the upper part in contact with the cold head. Therefore, the density of the working gas in the inner space of the pulse tube becomes large in the upper part and small in the lower part, and the working gas generates convection by the influence of gravity. As a result, the working gas in the lower part in contact with the high temperature end rises and transmits heat to the cold head disposed at the upper part, and the working gas in the upper part in contact with the cold head transmits cold heat to the high temperature end disposed in the lower part, so that heat loss occurs, and the refrigeration output of the cryocooler is lowered.
On the other hand, in an installed state, where the cold head of the pulse tube is positioned to be lower than the high temperature end, the temperature of the working gas in the inner space of the pulse tube becomes such that the temperature of the upper part in contact with the high temperature end is high as compared with the lower part in contact with the cold head. Therefore, density of the working gas in the inner space of the pulse tube becomes large in the lower part and small in the upper part. Accordingly, in this installation posture, since the working gas does not generate convection, due to gravity, and the heat loss by the convection can be neglected, a high refrigeration output can be obtained.
Embodiments of the present invention are directed to solve the aforementioned difficulties of the conventional pulse tube cryocooler, as described above. In addition, embodiments of the present invention provide a pulse tube cryocooler in which a difference in a refrigeration output due to a difference in an installation posture is reduced, such that a stable refrigeration output can be obtained even under various installation conditions.
To solve the above-described problems, it is an aspect of the present invention to provide a pulse tube cryocooler having a high refrigeration output compared to conventional cryocoolers by setting space volumes, lengths, cross sections, etc., of a regenerator and a pulse tube at specific ratios.
Additional aspects and advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.
Accordingly, to achieve the above and other aspects, an embodiment of the present invention includes a pulse tube cryocooler having a compressor to repeatedly feed and suction a working gas, a regenerator coupled to this compressor through a heat radiation part and having a regenerating agent, a pulse tube coupled to the regenerator through a cooling part, and a buffer tank coupled to the pulse tube through a heat radiation part and an inertance tube. This pulse tube cryocooler can be characterized in that a ratio of a space volume of the pulse tube to a space volume of the regenerator becomes 0.75 to 1.5.
By setting the ratio of the space volume of the pulse tube to that of the regenerator in this range, from the viewpoint of the indicated refrigeration output, the pressure amplitude generated in the compressor can be efficiently transmitted to the pulse tube, occurrence of the heat loss can be suppressed, and the refrigeration efficiency can be improved.
The ratio of a length of the pulse tube to a length of the regenerator may be 0.9 to 1.9. Such that, since the loss of the pressure amplitude generated in the compressor can be further lowered, the refrigeration efficiency of the cryocooler is improved, and the refrigeration output can be raised.
To achieve the above and other aspects, an embodiment of the present invention includes a pulse tube cryocooler having a compressor to repeatedly feed and suction a working gas, a regenerator coupled to this compressor through a heat radiation part and having a regenerating agent, a pulse tube coupled to the regenerator through a cooling part, and a buffer tank coupled to the pulse tube through a heat radiation part and an inertance tube, and characterized in that when a diameter of a circle having an area equal to an inner cross section of the regenerator is made an inner diameter, a value obtained by dividing a length of the regenerator by a square of the inner diameter becomes 0.11 to 0.26.
By setting the length of the regenerator and the cross section in this range, the heat resistance in an axial direction in the regenerator can be made large to suppress a heat loss, while suppressing a pressure loss so that a pressure amplitude generated in the compressor can be efficiently transmitted to the pulse tube, thereby making it possible to raise the indicated refrigeration output and to improve the refrigeration efficiency.
To achieve the above and other aspects, an embodiment of the present invention includes a pulse tube cryocooler where conditions under which a difference in refrigeration output due to an installation posture can be reduced as compared with conventional pulse tube cryocoolers.
In an additional embodiment, in a pulse tube cryocooler having a pulse tube and a regenerator linearly disposed, a ratio of an inner cross section of the pulse tube to an inner cross section of the regenerator may be set to not less than 0.1 and not higher than 0.35.
Alternatively, in another pulse tube cryocooler embodiment, the inner diameter of the pulse tube may be set to 12 mm or less.
Thus, when a pulse tube cryocooler, according to embodiments of the present invention, is constructed such that a ratio of an inner cross section of the pulse tube to an inner cross section of the regenerator, or if the inner diameter of the pulse tube is set to be within particular ranges, the heat loss due to natural convection of the working gas generated in the inner space of the pulse tube can become equivalent to the heat loss due to natural convection of the working gas generated in the inner space of the regenerator, even if the installation posture is changed up and down. Since the heat losses of both are cancelled, the difference in the refrigeration output caused by the installation posture can be reduced, and the pulse tube cryocooler with a stable refrigeration output can be obtained under various installation conditions.